As technology development and demands for mobile devices increase, demands for secondary batteries as energy sources are rapidly increasing. Among the secondary batteries, lithium secondary batteries having high energy density and working potential, a long cycle life, and a low self-discharge rate have been commercialized and widely used.
Further, with growing concerns about environmental issues, many researches have been conducted on electric vehicles and hybrid electric vehicles which may be employed in place of fossil fuel-based vehicles such as gasoline vehicles, diesel vehicles, etc., which are one of major causes of air pollution. Use of lithium secondary batteries having high energy density and discharge voltage as power sources for such electric vehicles and hybrid electric vehicles has been actively studied, and commercialization thereof has been also actively carried out.
As a positive electrode material for the lithium secondary battery, LiCoO2, a ternary system material (NMC/NCA), LiMnO4, LiFePO4, etc. is currently used. Of them, LiCoO2 has problems in that cobalt is expensive and LiCoO2 has low capacity at the same voltage, as compared with ternary system materials. Therefore, use of ternary system materials is gradually increasing in order to increase the capacity of secondary batteries.
LiCoO2 has excellent physical properties such as high rolling density, etc., and excellent electrochemical properties such as high cycling property, and therefore, it has been frequently used until now. However, since LiCoO2 has a charge/discharge current capacity as low as about 150 mAh/g, and its crystal structure is unstable at a high voltage of 4.3 V or more, it has problems of a rapid reduction of lifetime property and ignition caused by reactions with an electrolyte.
In particular, when a high voltage is applied to LiCoO2 in order to develop a high capacity secondary battery, Li usage of LiCoO2 is increased, which may increase possibility of surface instability and structural instability.
To solve these problems, a method of doping or coating the surface of LiCoO2 with a metal such as Al, Ti, Mg, or Zr is generally suggested. However, there are restrictions on the methods, structural stability is still poor, sufficient lifetime characteristics are not obtained, or capacity deterioration occurs.
Accordingly, it is highly necessary to develop a lithium cobalt oxide-based positive electrode active material which may be stably used without deterioration of battery performances at high voltages.